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I seek to put a layer of fluorine on a rotationally molded cross-linked polyethylene (XPE) plastic gasoline tank. I sell tanks and they will be more eco-friendly if I can accomplish this.

I am an electrical engineer and my last experience with chemistry was Chem 200 lecture and lab at SDSU, a one semester intensive "all of chemistry for engineers" kind of course. I asked a chemistry professor at my alma mater about this, who said:

Xenon difluoride is a powerful fluorinating agent, and is soluble (according to Wikipedia) in anhydrous methyl cyanide and several fluorinated solvents. Offhand, this looks to me like it would do the job, but it's not a friendly compound (toxic, possibly corrosive).

The concept is already in place all around us: any time you buy a plastic jug with harsh chemicals in it, the jug has been fluorinated. So far, the majority of my learning is confirmation that yes, indeed, fluorinating polyethylene (PE) surfaces with $\ce{XeF2}$ leads to decreased gas permeability.

I've done a bit of Googling and bought a couple of great textbooks, Fluoropolymers 1; Synthesis and Fluoropolymers 2; Properties. Fluoropolymers 1 seems to have great stuff in a couple sections. Perusing Fluoropolymers 2, I haven't seen anything jump out at me yet.

There are two methods I have read about:

  1. Involves an oven, steady high temperature, and plasma discharge.
  2. "Surface Fluorination of Polymers Using Xenon Difluoride", or Gaseous $\ce{XeF2}$ and PE are placed in a "stainless steel reactor" and left alone.

Seeing the options, the 2nd seems the way to go, and this seems to be the epic equation concerning my quest quoted from Fluoropolymers 1 page 231, section 15.3:

The comparative accessibility and desirable properties of $\ce{XeF2}$ enable its use for the synthesis of fluoroorganic compounds under “mild” conditions. As a fluorinating agent $\ce{XeF2}$ can be used to effect electrophilic fluorination of various organic compounds with good yield and it is a useful reagent for generating radical cations. ... In many cases XeF2 is an attractive alternative to the conventional Balz–Schiemann reaction for the preparation of fluorinated aromatic compounds, e.g., fluorobenzenes:

$\ce{RC6H6 ->C[XeF2] Rc6H5F + Xe + HF ->C[XeF2] RC6H4F2 }$

Xenon difluoride reacts with carbon–carbon single, double, and triple bonds giving addition fluoroorganic compounds, e.g.,

$\ce{R-CH=CH2 ->C[XeF2] RCH2-CHF2 + Xe}$

and

$\ce{RCH2-CH3 ->C[XeF2] RCHF-CH3 + Xe + HF ->C[XeF2] RCF2-CH3 + Xe + HF}$

And this quote from page 236 gives me great hope:

Another argument in favor of the $\ce{XeF2}$ method is the evidence that since 1995 it has been used by a manufacturing company that had no previous experience with direct fluorination or fluorine at all.

... for I'm in the same position now!


How do I actually perform a fluorination surface treatment my XPE products?

Where / how do I go about getting $\ce{XeF2}$? And if it is soluble in a "fluorinated solvent" like my prof said, is there a way I can flush this through the interior of my tanks in a controlled manner that would accomplish some degree of fluorination?


Edited to add 10-22-12: I called a few companies today. Everyone is pointing me to pay Fluoro-Seal to do this. They are the only company providing this service in the US. I'll need to ship my products to them, wait a couple weeks, and pay $15~45 each tank. I'd still like some kind of bath or wash to do this. This is why no-one making replacement tanks has them emissions legal... all this triples my cost.


Edited to add 12-30-13: Fluoro-Seal has given up on fluorinating rotationally molded PE. I'm pushing a plastics manufacturer to develop their existing acetel product to work with rotational molding. Their solution for injection and blow molding is in production and impervious to fuels (they have parts in nearly all automotive fuel systems in the US right now). So there's hope for a solution; however it seems impossible to fluorinate PE.

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  • $\begingroup$ FWIW, I've been honing my Googlefu and finding articles by RJ Lagow from the 90s. Hopefully, somewhere he'll describe the physical environment to accomplish this, although my favorite would be a fluid I can "wash" it with. $\endgroup$ – Chris K Oct 21 '12 at 23:50
  • $\begingroup$ Update: This doctor's course notes were a boon for me: crab.rutgers.edu/~alroche/fluorocarbons.html $\endgroup$ – Chris K Oct 24 '12 at 8:13
  • $\begingroup$ "Offhand, this looks to me like it would do the job, but it's not a friendly compound (toxic, possibly corrosive)" - Find me a good fluorinator that isn't toxic or corrosive. The mechanism that makes fluorine gas, and fluorides of the groups on the right-hand side of the periodic table, so useful in reaction chemistry is the same mechanism that will kill you if you get a whiff of it, and that will start a roaring fire with materials you'd bet the rent on being nonflammable (like concrete, sand, asbestos, ceramics). $\endgroup$ – KeithS Apr 29 '13 at 22:22
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    $\begingroup$ You send it out to a commercial fluorinator, fluorine in nitrogen gas. Plastic engine oil jugs are typically surface fluorinated as a solvent barrier. If you go for acetal, remember that gasoline is 10% ethanol (courtesy of your idiot corrupt government) and other stuff, including detergent. Solvent attack. A gas tank must pass collision safety testing - be complaint rather than shatter on impact. $\endgroup$ – Uncle Al Feb 18 '14 at 5:54
  • $\begingroup$ @UncleAl Fluoro-Seal is the only company performing gaseous fluorinating of fuel tanks and they have determined they cannot process rotomolded PE tanks. $\endgroup$ – Chris K Feb 18 '14 at 23:43
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There is a simpler way to get fluorinated polyethylene.

Polyethylene is polymerized ethylene $\ce{CH2=CH2}$. As part of the polymerization, the double bond is no longer there. Polyethylene is essentially a very long saturated hydrocarbon: $\ce{(-CH2-CH2 -)}_n.$

enter image description here

Another polymer already exists that replaces all the hydrogen atoms with fluorine atoms: poly(tetrafluoroethylene) (PTFE), made from tetrafluoroethylene $\ce{CF2=CF2}$. PTFE is a long saturated ferfluorocarbon: $\ce{(-CF2-CF2 -)}_n.$

enter image description here

The most well known trademarked brand of PTFE is Teflon. Get your coated with or made out of Teflon/PTFE. It will be easier, cheaper, and safer in the long run. If XeF2 will fluorinate polyethylene (which is reasonably inert), imagine what it could do to yyou.

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  • $\begingroup$ Hi, thanks for your answer! Unfortunately, ptfe as an originating powder is 12x more costly. Next, teflon absorbs water, so the molder has to buy small batches, which makes it again 50% more costly. Last, teflon applications in the field as made by the OEM are absorbing gasoline molecules into the matrix, resulting in odd expansion (very large "bubbles"). PTFE isn't the source... If we could do multiple layer types, it could be considered in a layer, but that industry is 10x more costly than rotational molding. $\endgroup$ – Chris K Oct 21 '12 at 23:48
  • $\begingroup$ PTFE is very hydrophobic. It should not absorb water. $\endgroup$ – Ben Norris Oct 22 '12 at 11:14
  • $\begingroup$ I'm going off of what my vendor says. I don't doubt them when they got mad about losing a quarter ton of powder. $\endgroup$ – Chris K Oct 22 '12 at 21:11
  • $\begingroup$ Marking this as answered. Everyone is saying use nitrogen gas with 10% fluorine, which is "commercially available". There isn't an aqueous solution to my problem because of the difficulty getting the hydrogen off the surface of the polyethylene solid. $\endgroup$ – Chris K Oct 25 '12 at 21:49
  • $\begingroup$ BTW, just realized I had mistaken your suggestion of PTFE (teflon) with Nylon, PA6. I'll ask them about teflon. $\endgroup$ – Chris K Oct 30 '12 at 19:05
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There are commercial companies that fluorinate the surface of polyethylene. Its not clear whether they use fluorine, carbon tetrafluoride and plasma, or xenon tetrafluoride. Given the lower cost and ease of manufacturability of polyethylene compared to PTFE and the extreme hazards in casually using xenon tetrafluoride, I would suspect that utilizing a commercial service would be an optimal solution.

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